Heat transfer loops are often placed in the earth to provide for the heating and cooling of residential and commercial spaces. Since ground temperatures are generally similar to room temperatures in buildings, the use of such heat transfer loops can be cost effective alternatives to conventional heating and cooling systems. The installation of such heat transfer loops involves inserting a continuous loop of pipe connected to a heat pump unit into a hole or series of holes in the earth to act as a heat exchanger. A thermally conductive grout is then placed in the hole between the pipe wall and the earth. A heat transfer fluid can be circulated through the underground heat transfer loop to allow heat to be transferred between the earth and the fluid via conduction through the grout and the pipe wall. When the system is operating in a heating mode, a relatively cool heat transfer fluid is circulated through the heat transfer loop to allow heat to be transferred from the warmer earth into the fluid. Similarly, when the system is operating in a cooling mode, a relatively warm heat transfer fluid is circulated through the heat transfer loop to allow heat to be transferred from the fluid to the cooler earth. Thus, the earth can serve as both a heat supplier and a heat sink.
In heat pump systems that are ground sourced, closed loops are often used to exchange heat between the ground and a conditioned space such as an office building or residential house. In certain cases, such as retrofit installations, horizontal directional drilling (“HDD”) of boreholes may be an economical way to add ground source heating and cooling to an existing structure. Horizontal boreholes may be drilled under the existing structures without disturbing the building above.
The efficiency of the heat transfer loop is affected by the grout employed to provide a heat exchange pathway and a seal from the surface of the earth down through the hole. The grout needs to have a relatively high thermal conductivity to ensure that heat is readily transferred between the heat transfer fluid and the earth. Further, the grout must form a seal that is substantially impermeable to fluids that could leak into and contaminate ground water penetrated by the hole in which it resides. The hydraulic conductivity, which measures the rate of movement of fluid (i.e., distance/time) through the grout, is thus desirably low. Moreover, the grout needs to have a relatively low viscosity to allow for its placement in the space between the heat transfer loop and the earth without leaving voids that could reduce the heat transfer through the grout. In an attempt to achieve such properties, grouts containing sand to enhance their thermal conductivity have been developed that are extremely labor intensive to prepare. In particular, conventional grouts often require several hundred pounds of sand to render them suitably thermally conductive. Unfortunately, the thermal conductivity that may be achieved by these conventional grouts is limited by the amount of sand that can be incorporated into and properly suspended in the grout. Also, the preparation of such grouts is inflexible in that the concentrations of the components and the mixing procedures must be precise to avoid problems in the field.
Many of the existing grouts for horizontal heat loops require a specialized positive displacement pump like a progressing cavity, rotor-stator style pump. Pumps that may be readily available at HDD sites, such as centrifugal and piston pumps, are typically not recommended. These conventional grouts may also have a low thermal conductivity and may also require the use of a tremie line during installation.
Therefore, a need exists for a thermally enhanced grout for use in sealing a heat transfer loop to the earth. In addition to the grout to have a higher thermal conductivity than conventional grouts, it is desirable that the grout be relatively easy and inexpensive to prepare, and may be installed using traditional grout pumps found on HDD rigs.